Steering systems which operate according to the steer-by-wire principle differ from conventional steering systems in that there is no mechanical link between the steering wheel and the steered wheel. Owing to the separateness of this steering system, the torque applied by virtue of the steering column to the steering wheel in a conventional systems is absent. The steering sensation previously determined by the road, the tires, the chassis and the transmission behavior, is instead produced by a steering wheel actuator. The forces and torques detected by one or more force and torque sensors in the steering linkages of the steered wheels are input to a central control unit. Via a steering wheel actuator, a reaction is passed on to the driver as a function of the operating conditions at the time, and this reaction being similar to the forces and torques that occur with a conventional steering system.
During driving, force and torque sensors in the steering linkages of the steering wheels deliver force signals which are proportional to the steering forces occurring at the steered wheels. The highest forces and torques are detected, evaluated by a control unit, and passed on to the steering wheel actuator to produce a torque on the steering wheel. The steering wheel actuator is controlled in such a manner that the turning resistance or torque variation, perceived by the driver, corresponds to that which would be provided by a conventional steering system when equivalent force and torque effects occur in the steering linkage.
To be able to produce an artificial reaction at the steering wheel actuator, the steering tie rod force occurring as a wheel reaction must be detected. This measured parameter then serves to regulate the necessary steering torque at the steering wheel.
From the prior art it is known, in steering systems comprising an electric motor in the steering actuator, to determine the steering tie rod forces by measuring the motor current. In steering systems with hydraulic steering actuators, the steering tie rod forces are determined by a pressure measurement. In the case of low steering tie rod forces, such as those that occur when driving straight ahead, however, the disadvantage is that the measurements of the motor current or pressure occurring react insensitively because of friction and damping influences, and this so leads to an inaccurate result. The consequence of this is that the centering sensation, passed on to the driver, is poor which, in turn, leads to insecure driving behavior.
The purpose of the present invention is to present a steering system in which even small changes of the steering tie rod forces are passed on to the driver in such manner that he or she experiences a more secure driving sensation.
The objective of the invention is achieved by a steering system of the type described.